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It is a security thing.
If the method is not POST or if the HTTP Referer check fails, XOOPS will not allow $xoopsDB->query() with anything but SELECT statements.
If you are in a secure environment, where you can trust the SQL query even though it is not submitted via POST and/or it is not with a valid HTTP Referer, you can use $xoopsDB->queryF() instead. This will go through regardless, but should only be used in these circumstances.